Sensitivity equalizing circuit for control apparatus



June 14, 1949. R. F. WILD SENSITIVITY EQUALIZING CIRCUIT FOR CONTROL APPARATUS 2 Sheets-Sheet 1 Filed Oct. 6, 1945 OSCILLATOR R. F. AMPLIFIER DISCRIMINATOR VOLTAGE AMP. POWER AMP.

FIG.

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Jun; 14, 1949. R. F. WILD SENSITIVITY EQUALIZING CIRCUIT FOR CONTROL APPARATUS 2 Sheets-Sheet 2 Filed Oct. 6, 1945 mm B INVENTOR.

RUDOLF F. WILD vvvvvvvv AAAAA v ATTOR NE Patented June 14, 1949 SENSITIVITY EQUALIZING CIRCUIT FOR CONTROL APPARATUS Rudolf F. Wild, Philadelphia, Pa., assignor, by mesne assignments, to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application October 6, 1945, Serial No. 620,831

14 Claims.

The present invention relates to follow-up systems of the frequency discriminating type disclosed in my copendin application, Serial No. 537,505, filed on May 26, 1944, and more particularly, to the provision in such systems of means to equalize the sensitivity of response over the operating range for the purpose of obtaining smooth and uniform operation thereof over a wider range of operation than has heretofore been possible.

In follow-up systems of the type disclosed in my prior application referred to above, no especial difficulty is encountered due to inequalities in the system sensitivity over the operating range when the range through which the frequency is varied is suitably narrow. It is desirable in many applications, however, to vary the frequency over a much wider range to minimize the effects of frequency drifts caused by ambient temperature variations and other unpredictable factors. When the range of frequency variation is widened so as to minimize the effects of such drifts, the follow-up systems of the type disclosed in my said prior application are characterized by the wide differences in their system sensitivities in various portions of the range of operation, the differences being so pronounced that a tendency for the occurrence of chattering or hunting may exist at one portion of the range of operation, while an excessive dead spot may simultaneously exist in another portion. Insofar as I am aware, I am the first to have provided a, practical solution for the difficulties in such follow-up systems resultin from variation over the operating range of the system sensitivity.

Accordingly, an object of the invention is to provide in follow-up systems of the type disclosed in my prior application, means operative to substantially equalize the system sensitivity over the range of operation in order to obtain smooth and uniform operation over a wide operating range. A more specific object of the invention is to provide simple and efficient means for accomplishing this result.

The various features of novelty which characterize this invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and specific objects obtained with its use, reference should be had to the accompanying drawings and descriptive matter in which is illustrated and described a preferred embodiment of the invention.

Of the drawings:

Fig. 1 is a diagrammatic illustration of one embodiment of the invention;

Fig. 2 is a wiring diagram showing the electrical circuit arrangement of Fig. 1;

Fig. 3 is a graph illustrating the variations in sensitivity of the frequency discriminator of Fig. 2 over a range of frequency variation; and

Fig. 4 is a graph showing the manner in which the sensitivities of the transmitter and receiver portions of the arrangement of Fig. 2 are made to vary to provide a substantially constant over-all sensitivity.

In Fig. 1 I have illustrated more or less diagrammatically a system for recording the rate of flow of a fluid through a pipe or conduit l. The rate of flow of fluid through the pipe I is detected by a manometer generally designated at 2 which is arranged to operate a variable condenser 3 for detuning a resonant electrical circuit in which the variable condenser is connected. Specifically, the detuning means or variable condenser 3 is electrically connected to and arranged to control the operation of electronic apparatus designated generally by the reference character 4.

The electronic apparatus 4 includes an oscillator, a radio frequency amplifier, frequency discriminating means, a voltage amplifier and a power amplifier. A preferred form which the electronic apparatus 4 may take is illustrated in detail in Fig. 2. The electronic apparatus 4 is arranged to control the selective energization for rotation in one direction or the other of a reversible electrical motor generally designated at 5. As shown, the motor 5 is of the rotating field induction type and is arranged to operate retuning means designated by the reference character 6 for accomplishing a follow-up or rebalancing action on the electronic apparatus 4. Motor 5 also operates indicating and recording mechanism generally designated at I. It will be understood that, if desired. the recording mechanism 1 may be arranged to operate control means, not shown, 1

for regulating the flow of fluid through the pipe I.

The manometer 2 for ascertaining the rate of. flow of fluid through the pipe I may be of any known type and, as shown, includes an orifice plate 8 which is positioned in the pipe I for the purpose of creating a pressure differential which varies in accordance with the rate of flow of fluid through the pipe. The pressure differential so produced varies in accordance with the square of the rate of flow through the pipe. Manometer 2 also includes a high pressure chamber 9 which is connected by a tube It to the high pressure side of the orifice plate 8 and includes a low pressure chamber II which is connected by a tube l2 to the low pressure side of the orifice plate 8. The low pressure chamber II and the high pressure chamber 9 communicate with each other through a tube I3.

The relative levels of mercury or other suitable liquid located in the high and low pressure chambers 9 and II vary in accordance with the difference in the pressures within those chambers, and consequently, provide a measure of the rate of fluid flow through pipe A member H which floats on the mercury in the high pressure chamber 9, and hence, rises and falls in accordance with the variations in pressure differential in the two chambers 9 and II, is arranged to angularly deflect a gear sector |5. The gear sector l5 meshes with a gear |'6 which is arranged to operate the detuning means 3 comprising movable condenser plates I! which are deflected relatively to stationary condenser plates |8 on angular deflection of the gear sector l5. As shown in Fig. 1, an increase in the rate of fluid flow through the pipe I causes the condenser plates H to rotate in a clockwise direction to decrease the capacitance between the condenser plates H and I8.

While the detuning means 3 in the arrangement of Fig. 1 has been shown as being varied in capacity in accordance with the changes in flow of fluid through a pipe, it will be understood that such changes in capacity may be made in accordance with the changes in any variable condition which may be translated into a movement or deflection corresponding in magnitude and direction to the changes in the condition it is desired to measure and/or record. For example, the changes in capacitance between the condenser plates I! and I8 may be effected in accordance with the movements of a stylus in one coordinate on a plotting table, as disclosed in my copending application, Serial No. 620,832, filed concurrently herewith.

The reversible electrical motor 5 includes a stator l9 and rotor 20 which is equipped with suitable conductor bars. The stator I9 is provided with a power winding 2| and with a control winding 22. Depending on the phase relation of the electrical current flow through the power winding 2| to that through the control winding 22, the rotor 20 is actuated for rotation in one direction or the other to cause rotation of a pinion gear 23 in a corresponding direction. The pinion gear 23 drives a gear 24 which is carried by a shaft 25 and is provided with a projection 26 arranged to abut against the pinion gear 23 for the purpose of limiting the extent of rotation of the gear 24.

The gear 24 carrier a cable drum 21 which operates a cable 28 strung over pulleys 29, 34, and 3| and a cable dru-m 32. The pulley 29 is carried by a lever 33 which is biased by a spring 34 in a ClOCkWlSe direction about a pivot 35 to maintain the cable 28 taut. The cable drum 32 is arranged to operate the retuning means 6 which, as shown, comprises a variable condenser having movable condenser plates 36 adapted to be rotated with respect to relatively stationary condenser plates 31 upon rotation of cable drum 32. The retuning means 6, therefore, is adjusted in accordance with the angular positions assumed by the rotor 2|) of the motor 5.

The shaft 25 which carries the gear 24 may operate an indicating pointer with respect to a suitably calibrated indicating scale, not shown. Also mounted on the shaft 25 is a gear 38 which meshes with a gear sector 39 so that upon operation of the motor 5 the sector 39 is rotated about its pivot 40. The gear sector 39 positions a pen arm 4| with respect to a slowly rotating chart 42 for the purpose of providing on the chart 42 a continuous record of the rate of fluid flow through the pipe I.

The wiring diagram of the electronic apparatus generally designated at 4 and controlled jointly by the variable condenser or detuning means 3 and by the variable condenser or retuning means 5, for selectively controlling the rotation and direction of rotation of the reversible motor 5, is shown schematically in Fig. 2. As seen in Fig. 2, the electronic apparatus 4 includes an electronic oscillator designated generally by the reference numeral 43, a stage of radio frequency amplification indicated at 44, frequency discriminating means designated by the reference character 45, a voltage amplifier and limiter indicated at 46, and a power amplifier indicated at 41. Electrical energy is supplied to the apparatus by means of a transformer 48. Thus, direct current energizing voltage, derived from the transformer 48 by means of a rectifier 49, is supplied to the oscillator 43, the stage of radio frequency amplification 44 and the voltage amplifier 46. The apparatus also includes a unidirectional motor 50 for rotating the chart 42 at a constant slow speed.

In the embodiment of the invention shown in Figs. 1 and 2, the detuning means 3 is utilized to control the frequency of oscillation of the high frequency current output of the oscillator 43. The oscillator high frequency current output is amplified by the radio frequency amplification stage 44 and the high frequency current output of the stage 44 is arranged to be modulated at a regular and appreciably lower frequency under control of the transformer 48. Changes in the frequency of oscillation of the high frequency current flow in the output circuit of the radio frequency stage 44 are detected by the frequency discriminating means 45 which is operative to produce a low frequency fluctuating output voltage when the frequency of the high frequency currents produced by the oscillator 43 and the frequency to which the frequency discriminating means 45 is tuned do not correspond. The fluctuating output voltage 50 created at the output terminals of the frequency discriminator 45 is of one phase or the opposite phase depending upon whether the frequency of the oscillator output current is higher or lower than the frequency to which the frequency discriminating means 45 is tuned. This fluctuating output voltage of low frequency is amplified and limited by the voltage amplifier and limiter 46. The amplified quantity is applied to control th power amplifier 41 which, in turn, regulates the flow of energizing current to the control winding of motor 5 and thereby controls the rotation and direction of rotation of the latter. Motor 5 operates to adjust the retuning means 6 for accomplishing a follow-up or rebalancing action on the apparatus and for operating the indicating and recording mechanism I.

Power is supplied to the apparatus through alternating current supply lines 5| and 52 from a source of alternating current, not shown, which supplies alternating current of a relatively low frequency. For purposes of illustration, the source of alternating current may be assumed to be the ordinary 60 cycle per second alternating current supply, although other frequencies of oscillation or alternation may be employed equally as well. A switch 53 controls the application of electrical power to the apparatus from the supply lines and 52.

The transformer 48, as shown, includes a low voltage primary winding 54 the terminals of which are connected across the line wires 5| and 52 through the switch 53, and also includes secondary windings 55, 56, 51, and 58. The secondary winding 55 is utilized to supply current to the heating filaments of the various electronic space discharge devices employed in the apparatus. The secondary winding 55 is employed to cause the high frequency current flow in the output circuit of the radio frequency stage 44 to be interrupted at the frequency of the voltage supplied by the alternating current supply lines 5| and 52, and is also utilized to supply, through the rectifier 49, direct current voltage to the oscillator 43, the radio frequency stage 44 and to the voltage amplifier 46. The secondary windings 51 and 58 cooperate with the power amplifier 41 for supplying energizing current to the control winding 22 of the motor 5.

The tube of rectifier 49 and the tube 88 utilized in the voltage amplifier 45, while shown separately in Fig. 2, may desirably be contained in one envelope. To this end, the tubes of rectifier 49 and the voltage amplifier 45 may each comprise one half of a commercially available type 7N7 tube. As shown, the rectifier tube includes anode, control grid, cathode, and heater filament elements. The heater filament is connected to and receives energizing current from the secondary winding 55 and the control grid is connected directly to the cathode. A filter designated generally at 59 is provided in association with the rectifier 49 for producing a direct current potential substantially free from ripple for energizing the output circuits of the oscillator 49, the stage of radio frequency amplification 44, and the voltage amplifier and limiter 46. i." Oscillator 43 is shown as being of the electron coupled type and includes a pentode tube! 50 hich may be of the commercially available t 6SJ'7. The tube 60 includes an anode, a suppressor grid, a screen grid, a control grid, a cathode and a heater filament. The heater filament is connected to and receives energy from the transformer secondary winding 55. The control grid is connected through a resistor 9| to ground G and through a condenser 62 to one terminal of a parallel circuit including the detuning means or variable condenser 3 in one branch, and an inductance coil 64 in a second branch. The inductance coil 54 is inductively coupled to an inductance coil 65. The cathode of tube 59 is connected through the inductance coil 55 to ground G. The screen grid of tube 59 is connected through a condenser 65 to ground G andthrough a resistor 55 to the positive output terminal of the filter 59.

The oscillating circuit of the oscillator 43 1ncludes the control grid circuit of which the parallel circuit including the detuning means 9 forms a part, and also includes the screen grid circuit which may be traced from the positive terminal of the filter 59 through resistor 96 to the screen grid, the cathode, and inductance coil 55 to the grounded and negative side of the filter 59. These circuits are inductively coupled by the inductance coils 64 and 65 and provide for high frequency operation through a range of frequencies which, for purposes of explanation, may be assumed to vary from approximately 378 to 472 kilocycles depending upon the adJustment of the condenser 3.

The anode of tube 60 is electron coupled to the screen grid thereof so that the high frequency oscillating currents flowing in the screen grid circuit cause the voltage of the anode to oscillate at the same high frequency. The high frequency oscillating circuit including the anode may be traced from the positive output terminal of filter 59 through resistor Ii! to the anode of tube 60, the cathode thereof, and the inductance coil 65 to ground G. The suppressor grid of tube 50 is connected directly to ground G and serves the usual purpose of decreasing secondary emission from the anode.

The high frequency oscillating current so produced in the anode circuit of tube 50 is coupled by means of a condenser 68 and a resistor 69 to the input circuit of the radio frequency amplifying stage 44. Stage 44 includes a pentode tube 10 which may be of the commercially available type 6SJ7 and includes anode, suppressor grid, screen grid, control grid, cathode and heater filament elements. The heater filament is connected to and receives energizing current from the transformer secondary winding 55. The cathode is connected through a biasing resistor H which is shunted by a condenser 12 to the grounded terminal of resistor 69, and the junction of the latter resistor and condenser 58 is connected to the control grid of tube 79. The suppressor grid is directly connected to the cathode. Anode voltage is supplied tube 10 from the filter 59 through a circuit which may be traced from the positive filter terminal through the primary winding 13 of a transformer 14 to the anode of tube 10, the cathode thereof, and the parallel connected elements H and 12 to the grounded and negative side of the filter 59.

Energizing voltage is supplied the screen grid circuit directly from the transformer secondary winding 58 through a circuit which may be traced from the left end terminal of that winding, as seen in the drawings, through a resistor 15 to the screen grid, the cathode, and the parallel connected elements H and 12 to the other and grounded side of winding 55. The screen grid, as shown, is also connected by means of a condenser 15 to ground G. The magnitude of the alternating voltage impressed on the screen grid of tube 10 is of the proper value to cause the tube I0 to be rendered non-conductive during substantially the entire half cycle during which the screen grid is negative and to be rendered conductive during the other half cycle when the screen grid is positive. That is to say, the screen grid of tube 10 and the transformer secondary winding 55 are so utilized that the high frequency oscillation assumes its maximum amplitude near the beginning of each operative half cycle of the alternating voltage supply and continues at its maximum amplitude until near the end of that half cycle. The resistor 15 is included in the screen grid circuit to assist in the attainment of such operation and acts as a limiter to prevent the screen grid voltage from increasing beyond a predetermined value. In this manner, the screen grid voltage is made to approximate a square wave during the half cycles in which the screen grid is driven positive. Accordingly, the high frequency oscillations produced in the output circuit of the radio frequency amplifier stage 44 are maintained at approximately constant amplitude during the half cycles that the tube 10 is conductive and are zero during the other and alternate half cycles. For convenience of explanation, the half cycles in which the tube HI is conductive will be referred to hereinafter as the operative half cycles.

It has been found that amplitude modulation of the high frequency oscillation obtained in the manner illustrated and described is adequate for many uses of the present invention, but when it is desired or necessary to obtain amplitude modulation more closely approaching a square wave, gaseous discharge means as disclosed in my copending application, Serial No. 537,505, may be utilized.

The frequency discriminating means 45 includes the transformer 14 and a pair of diode rectifiers 11 and 18 which desirably may be contained Within a single envelope I9. Transformer 14 includes a split secondary winding in addition to the primary winding 13. One half of the split secondary winding has been designated by the numeral 88 and the other half by the numeral 8|. The center tap of the split secondary winding is connected through a blocking condenser 82 to the anode of tube 18 and also to the upper terminal of the primary winding 13. The said center tap is also connected to the point of engagement of a pair of resistors 83 and 84. If desired, an inductance coil or choke may be inserted in the last mentioned connection. The output voltage from the frequency discriminator 45 is obtained across the resistors 83 and 84.

As shown, a condenser 85 and a resistor 86 are each connected in parallel with the primary winding 13 of the transformer 14. The condenser 85 is provided to tune the primary winding 13 to a desired frequency and the resistor 86 is provided for loading purposes. The condenser 85 and resistor 86 are so selected as to effect equalization of the sensitivity of response of the entire follow-up system throughout its entire operating range for the purpose of obtaining smooth and uniform operation of the system throughout that range. The manner in which condenser 85 and resistor 85 operate to produce this desired result is explained in detail hereinafter.

The diode rectifiers 11 and 18 may be contained within a single tube such as the commercially available type 6H6. Each diode, as shown, includes an anode, a cathode, and a heater filament. The heater filaments are connected in series with each other and receive energizing current from the transformer secondary winding 55. The cathode of diode 11 is connected through resistor 83 to the junction of windings 88 and 8| and the cathode of tube 18 is also connected to that junction through resistor 84. The anode of diode 11 is connected to the end terminal of winding 80 while the anode of diode 18 is connected to the end terminal of winding 8|. A condenser 81 is connected in parallel with both of the resistors 83 and 84. The condenser 6 is connected in parallel with both of the transformer secondary windings 88 and 8| comprising the split secondary winding of the transformer 14, for tuning the said split secondary winding to a frequency within the operating range of 378 .to 472 kilocycles corresponding to the frequency of the high frequency oscillating currents produced by oscillator 43. A trimmer condenser 88' is also connected in parallel with the split secondary winding and is employed for the purpose of providing a fine adjustment of the zero setting of the instrument pen and pointer. Preferably, the condenser 88' is provided with a suitable knob or kerf to facilitate adjustment thereof. The blocking condenser 82 and the condenser 81 are so selected as to present low impedance to the high frequency oscillating currents flowing through them.

When the frequency of the oscillating current applied to the transformer primary winding 13 is the value to which the split secondary winding of the transformer is tuned, the voltages induced in the winding sections and 8| and appearing across the terminals of the split secondary winding will be out of phase with the applied primary voltage. This phenomena is one known in the art and requires no detailed explanation herein. For the case under consideration in which the oscillating voltage applied to the frequency discriminator is the frequency to which the split secondary winding is tuned, the voltage produced across the resistor 83 by the current flow through the diode rectifier 11 is of the same magnitude but of opposite polarity to the voltage produced across the resistor 84 by the current flow through the diode rectifier 18. Thus, the outputs of the two diode rectiflers 11 and 18 cancel each other and zero voltage appears across the terminals of the condenser 81.

If the high frequency oscillating signal impressed on the primary winding 13 of the transformer 14 has a frequency other than that to which the secondary winding is tuned, the phase relations of the oscillating currents in the primary and secondary windings of the transformer 14 are such that the outputs of the diode rectifiers 11 and 18 do not cancel and a direct current voltage appears across the terminals of the condenser 81. This voltage is of one polarity or the other depending upon whether the frequency of the high frequency oscillating signal applied to the transformer 14 is above or below the value for which the split secondary winding is tuned. The magnitude of the voltage depends upon-how much the frequency of the high frequency oscillating signal differs from the frequency value to which the split secondary winding is tuned.

Recalling now that the high frequency oscillating current which is applied to the discriminator 45 from the output circuit of the radio frequency amplifier stage 44 is an interrupted radio frequency wave, it will be seen that the output voltage produced across the terminals of the condenser 81 will be a pulsating direct current voltage whose polarity and magnitude are as noted above. Recalling further that the high frequency oscillating voltage in the output circuit of the radio frequency amplifier stage 44 is periodically interrupted at the frequency of the voltage supplied by the supply lines 5| and 52, it will be seen that the pulsations of the direct current voltage produced across the condenser 81 are of the same frequency as the voltage supplied by lines 5| and 52.

The pulsating direct current voltage so produced across the terminals of the condenser 81 may be considered as having two components, one a steady direct current voltage and the second an alternating current voltage. The alternating current component is amplified by the voltage amplifier and limiter 46 for controlling the power amplifier 41 and thereby the energization for rotation of the motor 5.

Voltage amplifier and limiter 48, as previously noted, comprises one section 88 of a twin type 7N7 tube, the other section of which forms the tube for rectifier 48. The section 88 includes an anode, a control grid, a cathode, and heater filament. Energizing current is supplied to the heater filaotalit' ii ROOM ment from the transformer secondary winding 55.

The input circuit of the triode section 98 is controlled in accordance with the pulsating voltage drop produced across the terminals of the condenser 81 and to this end the control grid is directly connected to the upper terminal of condenser 81 and the cathode is connected through a parallel connected resistor 89 and a condenser 98 to the lower end of the condenser 81. Direct coupling is chosen in preference to resistance-capacity coupling in order to minimize distortion of the square wave characteristic of the discriminator output voltage, although resistance-capacity couplin may be employed, if desired. It is noted that the lower terminal of condenser 81 is connected directly to ground G.

Anode voltage is supplied to the triode section 88 from the filter 59 through a circuit which may be traced from the positive terminal of the filter through a resistor 9| to the anode of triode 88, the cathode thereof, and the parallel connected elements 89 and 98 to the negative and grounded terminal of the filter 59.

The resistor 89 and condenser 98 serve to bias the control grid of the triode section 88 and are utilized for the purpose of maintaining the voltage of the control grid at a predetermined minimum value when the fluctuatin output voltage from the discriminator 45 is zero. This biasin circuit serves to provide biasing potentials as required for good amplification of small discriminator output voltages in excess of a predetermined amplitude, the triode section 88 acts as a limiter due to anode current saturation and cut off. In this manner, the characteristic of the voltage output from the discriminator 45, as seen in Fig. 3, of increasing in amplitude to a maximum value and then decreasing, is prevented from affecting the operation of the power amplifier 41 and thereby the motor 5.

The power amplifier 81 comprises a twin triode tube, such as a type 7N7 tube, including two triodes 92 and 93 in the same envelope. Each triode includes anode, control grid, cathode and heater filament elements. Energizing current is supplied the heater filament elements in parallel from the transformer secondary winding 55. The control grids of the triodes are connected directly to each other and to a contact 94 which is adjustable along the length of a resistor 95. The resistor 95 is connected in series with a condenser 96 from the anode of triode 88 of the voltage amplifier and limiter 48 to ground G. Condenser 95 is provided for impressing the fiuctuating component of voltage produced across the resistor 9| in the anode circircuit of tube 88 on the input circuit of the power amplifier I! while preventing the direct current component of the voltage across resistor 9| from being impressed on said input circuit. The signal from the voltage amplifier and limiter 48 thus is impressed simultaneous]; and equally on both of the input circuits of triodes 92 and 93. The adjustable contact 94 and associated resistor 95 are provided to facilitate adjustment in the gain of the power amplifier l1.

Anode voltage is supplied to the triodes 92 and 93 of the power amplifier from the transformer secondary windings 51 and 58, respectively. Specifically, the anode of triode 92 is connected to the left end terminal of winding while the anode of triode 93 is connected to the right end terminal of the winding 58. The cathodes of the triodes 92 and 98 are connected together and through a biasing resistor 91 to ground G. The adjacent terminals of the transformer secondary windings 51 and 58 are connected together and through the control winding 22 of the motor 5 to ground G, and hence, through the biasing resistor 91 to the cathodes of the triodes 92 and 93.

As has been previously stated, the reversible motor 5 is provided with a stator l9 having four pole pieces which are physically spaced apart by and also includes a squirrel cage motor 28 having interconnected bars. The power winding 2| is wrapped around two of the opposite pole pieces of the stator and the control winding 22 is wrapped around the remaining two opposite pole pieces. When only the power winding 2| is energized, the rotor 28 is not urged to rotation in either direction and remains stationary. When the cont ol winding 22 is energized and the voltage and current therethrough lead the voltage and current, respectively, in the power winding 2|, the rotor 28 is actuated for rotation in one direction, for example, in a clockwise direction. When the voltage and current in the control winding 22 lag the voltage and current, respectively, in the power winding, the rotor 28 rotates in the opposite direction.

Energizing current is supplied to the power winding 2| of the motor 5 through a circuit which may be traced from the alternating current supply conductor 5| through switch 53, condenser 98. the power winding 2|, and the switch 53 back to the supply conductor 52. The condenser 98 is so chosen with respect to the inductance of the power winding 2| as to provide a substantially resonant circuit when the rotor 28 of the motor is rotating at approximately full speed.

Power is supplied to the control winding 22 from the transformer secondary windings 57 and 58 through the anode circuits of the triodes 92 and 98 of the power amplifier circuit 41. through the circuits previously traced. A condenser 99 is connected in parallel with the control winding 22 and is so selected as to provide a parallel resonant circuit during both the stalled and running conditions of the motor. The transformer secondary windings 51 and 58 are so wound on the transformer 48 that the anode of one triode 92 is driven positive during one half cycle of the alternating voltage supplied by the conductors 5| and 52 and the anode of triode 98 is driven positive during the alternate half cycles.

When the frequency discriminator 45 is tuned to the frequency of the high frequency oscillat ing currents which are impressed thereon from the oscillator 43, and consequently the voltages produced across resistors 89 and 84 cancel each other, no alternating component of voltage is impressed on the control grids of triodes 92 and 98, and hence, these triodes are equally conductive during the respective half cycles in which they are arranged to conduct. The energizing current then supplied to the control winding 22 of the motor 5 from the power amplifier 41 includes a D. C. component and an A. C. com-.

ponent of twice the frequency of the voltage supplied by conductors 5| and 52. This energizing current is not effective to cause rotation of the rotor 28.

Upon change in the frequency of the high fre. quency oscillating currents applied to the frequency discriminator 45 from the frequency to which the split secondary winding is tuned, an alternating voltage having the same frequency as that of the voltage supplied by lines and 52 is Produced across the condenser 81. This alternating voltage, as previously noted, is of one phase or of opposite phase with respect to the voltage supplied by conductors 5i and 52 depending upon the direction of the change in frequency of the high frequency oscillating currents impressed on the frequency discriminator. This alternating voltage is amplified and limited by the voltage amplifier and limiter 46 and the amplified quantity, when impressed on the input circuits of triodes 92 and 93, causes a decrease in the conductivity of one triode and a corresponding increase in conductivity of the other triode. In consequence, energizing current is delivered to the control winding 22 of the motor 5 which is of one phase or of opposite phase relatively to the voltage supplied by conductors 5| and 52 depending upon which triode 92 or 93 has had its conduction increased. Therefore, the change in phase of the voltage derived from the frequency discriminator 45 is effective to cause reversal of the motor 5.

Inasmuch as the details of this motor drive circuit comprise no part of the present invention, further description thereof is thought unnecessary, particularly since those details are fully described in the copending application, Serial No. 421,173, filed December 1, 1941, by Walter P. Wills, which issued as Patent No. 2,423,540 on July 8, 1947.

In Fig. 3 there is illustrated a graph showing the output voltage characteristic of the frequency discriminator 45. The abscissa of the graph of Fig. 3 represents the frequency of the oscillating current supplied to the frequency discriminator while the ordinate represents the voltage output produced across the condenser 81. Curve A shows the discriminator output voltage characteristic obtained when the radio frequency voltage across the primary winding 13 of the discriminator is one value, graph B shows the discriminator output voltage characteristic obtained when said radio frequency voltage is a higher value, and graph C indicates the discriminator output voltage characteristic obtained when said radio frequency voltage is a still higher value. From these three graphs in Fig. 3, it can be readily seen that the output voltage from the discriminator increases with increasing radio frequency voltage across the discriminator primary winding 13.

The discriminator sensitivity, that is, the change in output signal with changing frequency, may be mathematically expressed as follows:

de (1) where D is the discriminator sensitivity, and dc is the change in discriminator output voltage across condenser 81 occurring upon a change df in the frequency of the oscillating current supplied to the frequency discriminator.

The overall sensitivity of the follow-up system shown in Figs. 1 and 2, or more specifically, the output signal obtained from the frequency discriminator 45 for a given or unit angle of rotation of the detuning condenser 3 may be stated mathematically as:

dE dE g where S is the overall sensitivity, dE is the change in output voltage from the frequency discriminator, d represents a. unit angle of rotation of condenser 3, and df is the change in frequency of the high frequency oscillating currents produced by oscillator 43 occurring when condenser 3 is rotated through said unit angle. In other words,

0 min.

Where L is the inductance of the parallel resonant circuit including the detuning condenser 3, and C min. is the minimum capacity of the condenser 3.

Hence, the rate of change of frequency per unit angle of rotation of the condenser 3, obtained by differentiating Equation 3, is:

df 1 dc 1 1 d 4, /Z d g KX a (4) where C is the total tuning capacity of condenser 3 and includes C min.

Since 1 f n/TO the rate of frequency change of the oscillating current produced by oscillator 43 upon unit angle of rotation of condenser 3 can also be written as:

Hence, the ratio of the rate of change of frequency at any operating point within the operating frequency range to the rate of change of frequency at the minimum frequency point is:

if: 3 a Cmax. f B 6 (Q g f min.

d fmin. C

It has been found that if the operating range of frequency is chosen between I min.=3'78 kilocycles and f max.=472 kilocycles, the relationship set forth in Equation '7 becomes practically linear over the entire range of frequency variation of the high frequency oscillating currents produced by oscillator 43, as is shown by curve D in Fig. 4 of the drawings.

As has been pointed out hereinbefore, it is desirable to maintain the overall system sensitivity substantially constant throughout the entire operating range. Since the discriminator primary circuit (including Winding 13) is resonant at a fixed frequency, the discriminator output voltage E for different frequencies within the operating range is a function of frequency, determined by the resonant frequency of the resonant circuit, and its damping, or, the ratio of reactance to resistance in the circuit. It should be remembered in this connection that the dis- 13 criminator primary circuit is tuned to a fixed frequency while the discriminator secondary circuit is tunable to all the frequencies within the operating range.

As is illustrated by Fig. 3, the discriminator sensitivity is a function of the radio frequency voltage developed across its primary circuit and increases with increasing primary voltage. While the sensitivity can be computed mathematically, the derivation becomes somewhat complicated since it is not suflicient to consider frequencies in the vicinity of the primary resonant circuit only, which simplification is always found in the literature on the subject.

Measurements show, however, that the discriminator sensitivity follows the decline of the primary discriminator voltage for frequencies increasingly different from the primary resonant frequency. It has also been found that the sensitivity versus frequency characteristic of the frequency discriminator approaches a hyperbolic character. Such a characteristic is most desirable and advantage is taken thereof in accordance with the present invention to make the discriminator sensitivity versus frequency curve inversely proportional to the frequency versus angle of condenser rotation curve, the latter of which, as shown by curve D of Fig. 4, is approximately a straight line,

For the purpose of matching these functions in order to obtain a constant product thereof, it is possible to adjust both the tuning of the discriminator primary circuit and the ratio of reactance to resistance of this circuit. The latter adjustment is accomplished by inserting a resistor in parallel relation with the parallel resonant circuit. As shown in Fig. 2, resistor 86 is connected in parallel with the primary winding 13 of the frequency discriminator 45 for this purpose.

Since the rate of change of frequency increases with increasing frequency, it is necessary that the discriminator sensitivity decrease with increasing frequency. Therefore, the discriminator primary circuit is tuned to a frequency below the lower limit of the operating range, and by way of illustration, may be tuned to a frequency of 325 kilocycles, when the operating range of frequency extends from 378 to 4'72 kilocycles. In a satisfactorily operating embodiment of the present invention, the resistor 86 was assigned a value of 7500 ohms. Those skilled in the art will understand that the proper primary resonant frequency and the proper value of the inserted resistor 86 can readily be found by experiment for any given range of frequency variation.

Curve E of Fig. 4 shows plotted against Curve D has a straight line characteristic which may be expressed mathematically as y=:r. From this it may be readily seen that the product of curves D and E is constant since the product of a: multiplied by is unity.

While the system described makes use of a minimum number of additional component parts for equalizing the system sensitivity throughout the entire operating range, it is noted that it is also possible to employ a Wide band frequency filter having a proper frequency response as the primary discriminator circuit, coupled by an untuned inductance link circuit to the discriminator secondary circuit. Obviously, any other means can be employed which are suitable for producing such a variation in primary voltage amplitude with frequency that the discriminator sensitivity varies inversely proportionally with the rate of change of frequency per unit angle of rotation of the detuning condenser 3. In all of these cases constant overall sensitivity is obtained throughout the operating range.

While this system has been described in connection with capacitance tuning it is noted that it is equally applicable to inductance tuning.

While in accordance with the provisions of the statutes, I have illustrated and described the best forms of the invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention as set forth in the appended claims, and that in some cases certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent, is as follows:

1. In a follow-up system, means for maintaining substantially constant the sensitivity of response of said system over the range of operation thereof comprising a controlling object, a controlled object, oscillation producing means controlled by said controlling object to produce electrical oscillations varying over a predetermined range of frequencies in accordance with the adjustment of said controlling object, the change in frequency of said oscillations for a given adjustment of said controlling object varying over the range of adjustment of said controlling object, a receiver for oscillations of said predetermined range of frequencies, variable tuning means, frequency discriminating means including a resonant circuit tunable by said tuning means over said predetermined range of frequencies, said frequency discriminating means being characterized in that it produces an output voltage of a magnitude which is substantially the same for a given change in the adjustment of said controlling object irrespective of the portion of the range of adjustment over which the latter is adjusted and including a primary circuit reactively coupled to said resonant circuit and tuned to a predetermined frequency outside said predetermined range of frequencies, and driving means controlled by the voltage produced by said frequency discriminating means to adjust said variable tuning means and said controlled object.

2. In an electrical system, meansfor maintaining substantially constant the sensitivity of response of said system over the range of operation thereof comprising adjustable oscillation producing means adapted to produce electrical oscillations varying over a predetermined range of frequencies in accordance with the adjustment of said oscillation producing means, the change in frequency of said oscillations for a given adjustment of said oscillation producing means varying over different portions of the range of adjustment thereof, variable tuning means, frequency discriminating means to which the oscillations are applied and including a primary circuit tuned to a predetermined frequency outside said predetermined range of frequencies and including a resonant circuit reactively coupled to said primary circuit and adapted to be tuned by said tuning means, said frequency discriminating means being characterized in that it produces an output voltage of a magnitude which is substantially the same for a given change in the adjustment of said oscillation producing means irrespective of the portion of the range of adjustment over which the latter is adjusted, and driving means controlled by the voltage produced by said frequency discriminating means to adjust said variable tuning means.

3. In a follow-up system, a variable frequency oscillation producing device which produces for the same adjustment different changes in the frequency of said oscillation over the range of adjustment, and a frequency responsive device responsive to the frequency of said oscillation, said frequency responsive device including means comprising a primary circuit tuned to a predetermined frequency outside the frequency range of adjustment of the oscillation producing device and a secondary resonant circuit reactively coupled to said primary circuit and tunable over the frequency range of adjustment of the oscillation producing device to compensate for the difference in the frequency change of said oscillation over the range of adjustment of said oscillation producing device to maintain a substantially constant system sensitivity.

4. In a follow-up system, an object to be controlled, a receiver for oscillations of a range of frequencies produced in accordance with the position of a controlling object, variable tuning means, drive means operative in response to variations in the received frequency to shift said tunin means; and said controlled object by an amount dependent upon said frequency variations, a voltage producing frequency variation response network on which the received oscillations are applied, said network tending to respond to a varying degree depending upon the position of the controllin object and including a primary circuit tuned to a predetermined frequency outside said range of frequencies and a secondary resonant circuit tunable by said tuning means over said range of frequencies and reactively coupled to said primary circuit whereby the degree of response of said network is made substantially independent of the position of the controlling object, and means responsive to the output voltage derived from said network to control the operation of .:aid drive means.

5. In a follow-up system, an object to be controlled, a receiver for oscillations of a range of frequencies produced in accordance with the position of a controlling object, variable tuning means, drive means operative in response to variations in the received frequency to shift said tuning means and said controlled object by an amount dependent upon said frequency variations, a freprimary resonant circuit on which the received oscillations are applied, said primary circuit being tuned to a predetermined frequency outside said range of frequencies, a secondary resonant circuit including a coil, said secondary circuit being tunable by said tuning means over said range of frequencies, means to reactively couple said circuits whereby the degree of response of said network is made substantially independent of the position of the controlling object, means to connect one end of said primary circuit to the midpoint of the coil of said secondary circuit, means to rectify the potentials at the two ends of said coil with respect to the other end of said primary circuit, and means to utilize the resulting direct current voltages to control said drive means.

6. In combination with a high frequency oscillator having means for adjusting the frequency thereof over a predetermined frequency range in a non-linear relation with respect to the position of a controlling object, a two-phase motor provided with a pair of energizing windings, variable tuning means operable by said motor, means to connect one of said windings to a source of alternating current, means to periodically interrupt the oscillation produced by said oscillator at the frequency of the alternating current supplied to said one motor winding, a frequency variation response network tending to respond to a varying degree depending upon the position of the controlling object and connected to the output circuit of said oscillator and adapted to produce an output voltage of one polarity or the other accordingly as the frequency of oscillation of said oscillator is higher or lower than the frequency to which said network is tuned, said network including a primary resonant circuit tuned to a predetermined frequency and a secondary resonant circuit reactively coupled to said primary circuit, said secondary circuit being tunable by said tuning means over a range of frequencies which does not include said predetermined frequency whereby the degree of response of said network is made substantially independent of the position of the controlling object, means responsive to the output voltage derived from said network to provide an alternating current through the other winding of said motor of the same frequency as the alternating current applied to said one motor winding, and means to cause said alternating currents to flow through said windings in phase quadrature.

7. In combination with a high frequency oscillator having means for adjusting the frequency thereof over a predetermined frequency range in a non-linear relation with respect to the position of a controlling object, a two-phase motor provided with a pair of energizing windings, means to connect one of said windings to a source of alternating current, means to periodically interrupt the oscillation produced by said oscillator at the frequency of the alternating current supplied to said one motor winding, a primary resonant circuit connected to the output circuit of said oscillator, said primary circuit being tuned to a predetermined frequency, a secondary resonant circuit including a coil, said primary and secondary circuits tending to respond to a varying degree dependin upon the position of the controlling object, means operated by said motor to tune said secondary circuit, said secondary circuit being tunable over a frequency range which does not include said predetermined frequency whereby the degree of response of said primary and secondary circuits is made substantially indepen- Q i i. r

dent of the position of the controlling object, means to reactively couple said circuits, means to connect one end of said primary circuit to the midpoint of said coil, means to rectify the potentials at the two ends of said coil with respect to the other end of said primary circuit to derive two direct current voltages, said direct current voltages being pulsating voltages having the same fre quency as the frequency of the alternating current applied to said one motor winding by virtue of the fact that the oscillating current applied to said primary resonant circuit by said oscillator is interrupted at that frequency, means to oppose said direct current voltages to derive an alternating component of voltage of the same frequency as the alternating voltage applied to said one motor winding and of one phase or of the opposite phase accordingly as one of said direct current voltages is of greater magnitude than the other indicative of the frequency of oscillation of said oscillator being higher or lower than the frequency value to which said secondary circuit is tuned, means controlled by said derived alternating component of voltage to provide an alternating current through the other winding of said motor of the same frequency as the alternating current applied to said one motor winding, and means to cause said alternating currents to flow through said windings in phase quadrature.

8. In combination with a high frequency oscillator having means for adjusting the frequency thereof over a predetermined frequency range in a non-linear relation with respect to the position of a controlling object, a two-phase motor provided with a pair of energizing windings, means to connect one of said windings to a source of alternating current, means to periodically interrupt the oscillation produced by said oscillator at the frequency of the alternating current supplied to said one motor winding, a primary resonant cir- .:uit connected to the output circuit of said oscillator, said primary circuit tending to respond to a varying degree depending upon the position of the controlling object and being tuned to a frequency lower than the lowest frequency value in said predetermined frequency range, means to lower the ratio of reactance to resistance of said primary circuit, a secondary resonant circuit including a coil, tuning means operated by said m tor to tune said secondary circuit, said secondary circuit being tunable by said tunin means over said predetermined frequency range whereby the degree of response of said primary circuit is made substantially independent of the position of the controlling object, means to reactively couple said circuits, means to connect the high alternating potential end of said primary circuit to the midpoint of said coil, means to rectify the potentials at the two ends of said coil with respect to the low alternating potential end of said primary circuit to derive two direct current voltages, said direct current voltages being pulsating voltages having the same frequency as the frequency of the alternating current applied to said one motor winding by virtue of the fact that the oscillating current applied to said primary resonant circuit by saidoscillator is interrupted at that frequency, means to oppose said direct current voltages to derive an alternating component of voltage of the same frequency as the altematil'lg voltage applied to said one motor winding and of one phase or of the opposite phase accordingly as one of said direct current voltages is of greater magnitude than the other indicative of the frequency of oscillation of said oscillator being higher or lower than the frequency value to which said secondary circuit is tuned, means controlled by said derived alternating component of voltage to provide an alternating current through the other winding of said motor of the same frequency as the alternating current applied to said one motor winding, and means to cause said alternating currents to flow through said windin s in phase quadrature.

9. In a frequency variation response network, a primary resonant circuit connected to a source of high frequency oscillating current variable over a predetermined range of frequencies, said primary circuit bein tuned to a predetermined frequency outside said range of frequencies, a secondary resonant circuit including a coil, said secondary circuit being arranged to be tuned over said range of frequencies, means to reactively couple said circuits, means to connect one end of said primary circuit to the midpoint of the coil of said secondary circuit, means to rectify the potentials at the two ends of said coil with respect to the other end of said primary circuit, and means to utilize the resulting direct current voltages.

10. In a frequency variation response network, a primary resonant circuit connected to a source of high frequency oscillating current variable over a predetermined range of frequencies, said primary circuit being tuned to a predetermined frequency lower than the lowest value of said range of frequencies, a secondary resonant circuit including a coil, said secondary circuit being arranged to be tuned over said range of frequencies, means to reactively couple said circuits, means to connect the high alternating potential end of said primary circuit to the midpoint of the coil of said secondary circuit, means to rectify the potentials at the two ends of said coil with respect to the low alternating potential end of said primary circuit, and means to utilize the resulting direct current voltages.

11. In a frequency variation response network, a primary resonant circuit connected to a source of high frequency oscillating current variable over a predetermined range of frequencies, said primary circuit being tuned to a predetermined frequency lower than the lowest value of said range of frequencies, means to lower the ratio of reactance to resistance of said primary circuit, a secondary resonant circuit including a coil, said secondary circuit being arranged to be tuned over said range of frequencies, means to reactively couple said circuits, means to connect the high alternating potential end of said primary circuit to the midpoint of the coil of said secondary circuit, means to rectify the potentials at the two ends of said coil with respect to the low alternating potential end of said primary circuit, and means to utilize the resulting direct current voltages.

12. In a frequency variation response network, a primary resonant circuit tuned to a predetermined frequency, a secondary resonant circuit tunable over a frequency range which does not include said predetermined frequency, said primary and secondary circuits being reactively coupled, connections between the primary and secondary circuits such that two alternating potentials of like polarity exist between the ends of said secondary circuit and one end of said primary circuit, one of said potentials being a maximum when the frequency of the applied oscillatin current is higher than the frequency value to which said sec ondary circuit is tuned, and the other of said potentials being a maximum when the frequency of the applied oscillating current is lower than the frequency value to which said secondary circuit is tuned, means to rectify the two potentials, and means to utilize the resulting direct current voltages.

13. In a frequency variation response network, a primary resonant circuit tuned to a predetermined frequency, a secondary resonant circuit tunable over a frequency range the lowest frequency value of which is higher than said predetermined frequency, said primary and secondary circuits being reactively coupled, connections between the primary and secondary circuits such that two alternating potentials of like polarity exist between the ends of said secondary circuit and the low alternating potential end of said primary circuit, one of said potentials being a maximum when the frequency of the applied oscillating current is higher than the frequency value to which the secondary circuit is tuned, and the other of said potentials being a maximum when the frequency of the applied oscillating current is lower than the frequency value to which said secondary circuit is tuned, means to rectify the two potentials, and means to utilize the resulting direct current voltages.

14. In a follow-up system, a variable frequency signal producing device which produces for the same adjustment different changes in the frequency of said signal over the range of adjustment, and a frequency responsive device responsive to the frequency of said signal, said frequency responsive device including electrical circuit means having a first portion tuned to a frequency outside of said range of adjustment and having a second portion coupled to said first portion and tunable over said range of adjustment to compensate for the difference in the frequency change of said signal over the range of adjustment of said signal producing device to maintain a substantially constant system sensitivity.

RUDOLF F. WILD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,888,721 Goldsborough Nov. 22, 1932 2,163,234 Case June 20, 1939 2,379,689 Crosby July 3, 1945 2,396,091 DeBey Mar. 5, 1946 2,404,344 Wild July 16, 1946 Certificate of Correction Patent No. 2,473,401 June 14, 1949 RUDOLF F. WILD It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 58, for carrier read carries; column 5, line 43, for tube 50 read tube 60; column 9. line 33, after voltages and before in insert the period and words For discriminator output voltages; column 10, line 11, for motor read rotor;

H a ,7 column 12, lines 53 and 54, equation 7, for read dqfi f 4 :mm. 91:: f e d min. 1min. and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 21st day of February, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,473,401 June 14, 1949 RUDOLF F. WILD It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 58, for carrier read carries; column 5, line 43, for tube 50 read tube 60; column 9. line 33, after Voltages and before in insert the period and words For dzscrt'minator output voltages; column 10, line 11, for motor read rotor;

3 ,7 column 12, lines 53 and 54, equation 7, for )(Qg m1 read and that the said Letters Patent shouldbe reud with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 21st day of February, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

